Programmable window: a device for controlling the opacity of small-scale areas within a large scale transparent membrane

ABSTRACT

Natural lighting profiles are affected in a building facade and internal transparent membrane using a transparent sheet having one or more pixel elements, wherein each of the pixel elements is capable of being driven to control opacity (by shading). A programmable controller is used to send control commands to control opacity of the pixel elements, wherein the percentage of light transmitted through the display panel forming part of the facade is controlled via the controller.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to the field of environmentalcontrol and transparent displays. More specifically, the presentinvention is related to a programmable electronic device for controllingthe opacity of small-scale areas within a large-scale transparentmembrane for applications in architecture.

2. Discussion of Prior Art

The current range of possibilities for displaying information on abuilding-scale includes all forms of static and illuminated signs andpanels, projections, as well as building-sized LED panels and onedirectional OLED display arrays. The problem with most of the above isthat the display device completely blocks light. For instance, in thecase of facade displays such as those in Times Square, N.Y., there is noview or natural light inside the buildings used as displays, andsimilarly in the case of window scale displays. In the case of OLEDdisplay arrays, while light can travel across them, there is noefficient method for regulating the amount of light that does gothrough.

For the purpose of dynamic environmental control there is also existingwork in the field of Liquid Crystal (LC), Suspended Particle Devices(SPD) and Electrochromic Devices (ECD). These are used to control theamount of light that crosses windows, mirrors, and alike in a variety ofapplications. No work has been done in pixellating these materials toproduce a large scale display and accurate environmental control devicewith architectural applications.

SUMMARY OF THE INVENTION

The present invention provides for a device to control the opacity ofsmall areas within a large transparent membrane. The transparentmembrane of the present invention can be used both as a display panelincorporated onto any building facade or interior membranes, and forenvironmental control, namely as a tool to accurately and selectivelycontrol the amount of light that crosses small areas within thetransparent envelope of an architectural volume. In both cases thedevice will display patterns that could both be looked at, and lookedthrough.

When used as a display device on a whole/or part of a building's facade,the present invention's window sections will appear to be merelychanging in opacity as viewed from the inside of the building, withoutblocking the outside view entirely. From an external viewpoint, theaggregate set of window panels could display discernible information.When used for environmental control, lighting levels will be controlledin a non-uniform way within an architectural space, leading to highdegrees of accuracy. Also, while appearing well regulated on the inside,the image formed on the outside of the building might also contribute toits aesthetic characteristics.

The nature of the programmable window also allows for great freedom inchoosing the “pixel” size and shape. This elemental unit which changesopacity within the window can be custom designed in order to be of anyshape and size, so that an architect or designer can achieve the desiredvisual effects with great flexibility.

The present invention aims to present several applications for theprogrammable window, offers a method for incorporating it into anarchitectural environment, and suggests a method to construct such awindow and control the pixel elements within it.

The present invention provides for a display system used in a buildingfacade comprising a display panel placed as part of the facadecomprising one or more pixels, each of the one or more pixels capable ofbeing individually driven to control opacity, wherein the voltagesupplied to each pixel is controlled to vary the shading of that pixel,resulting in a variance of the percentage of light transmitted throughthe display panel forming part of the facade.

In one embodiment, the display system is used in environmental controlwith lighting levels controlled in a non-uniform manner within anarchitectural space.

The present invention also provides for a display system used in abuilding facade comprising: a display panel placed as part of the facadecomprising one or more pixels, each of the one or more pixels capable ofbeing driven to control opacity; and a controller programmable to driveeach of the one or more pixels to control the shading of the pixel,wherein percentage of light transmitted through the display panelforming part of the facade is controlled by the controller.

The present invention also provides for a display system used in abuilding facade, wherein the system in this embodiment, comprises: adisplay panel placed as part of the facade comprising a matrix of one ormore pixels under direct drive, with each element in the matrix beingindependently driven to control opacity of that element; and acontroller programmable to drive each of the one or more pixels tocontrol the shading of the pixel, wherein percentage of lighttransmitted through the display panel forming part of the facade iscontrolled by the controller.

The present invention also provides for a display system used in abuilding facade, wherein the present invention, in this embodiment,comprises: a display panel placed as part of the facade comprising amatrix comprising one or more pixel elements with common anode rows andcommon cathode columns each assigned an independent gating element; anda controller programmable to drive each of the one or more pixels tocontrol the shading of the pixel, wherein percentage of lighttransmitted through the display panel forming part of the facade iscontrolled by the controller.

The present invention also provides for a display system used in abuilding facade: a display panel placed as part of the facade comprisinga display array comprising a plurality of pixel elements driven viamulti-line addressing; and a controller programmable to drive thedisplay array to control the shading of elements of the array, wherein acolumn in the display array is handled by the controller at any giventime, wherein percentage of light transmitted through the display panelforming part of the facade is controlled by the controller.

The present invention also provides for a method to affect naturallighting profiles in a building facade comprising: implementing adisplay panel of a facade using one or more pixel elements, each of theone or more pixels capable of being driven to control opacity; receivingcontrol commands to control opacity of at least one pixel element;controlling each of the one or more pixels to control the shading of thepixel according to the received control commands, wherein the percentageof light transmitted through the display panel forming part of thefacade is controlled based on the control commands.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-d illustrates the present invention's four main applicationswithin architecture.

FIG. 2 illustrates window modules used in a building facade as per theteachings of the present invention.

FIG. 3 a large-scale application developed based on the teachings of thepresent invention.

FIG. 4 illustrates the decibel (dB) gain responses for the two SPD pixelsizes in series with a 100k resistor of 1% tolerance.

FIG. 5 illustrates a basic dual N-channel half-bridge driver using abootstrap method for high-side gating.

FIG. 6 illustrates a CMOS half-bridge driver using a simple RC gatedrive.

FIGS. 7 a-b illustrate bi-polar pulsing of transparency variable.

FIGS. 8 a-b illustrate transparency variable with continuous drive underan equivalent voltage level and frequency.

FIG. 9 a illustrates the multi-line strobing strategy for a 2 by 2 pixelarray.

FIG. 9 b illustrates an ideal waveform applied to P1.

FIG. 9 c illustrates an ideal waveform applied to P2.

FIGS. 10 a-b illustrate passive matrix addressing and cross-talk.

FIG. 11 illustrates crosstalk during multi-line addressing, occurringwhen row pixels are assigned different pulse widths.

FIGS. 12 a-b illustrate multi-line addressing and cross-talk.

FIG. 13 illustrates an ideal response of the SPD pixel to be able todriven without passive addressing.

FIG. 14 illustrates an anti-crosstalk strategy using unidirectionaldrive and reverse conduction blocking diodes.

FIGS. 15 a-b illustrate image analysis to study relative pixelbrightness under direct addressing and MLA strobing.

FIGS. 16 a-b illustrate active and inactive pixels underdirect-addressing and MLA strobing.

FIG. 17 illustrates a 2 by 2 section depicting the active matrixAC-drive wiring topology, wherein the column drivers are themselvescomposed of independent drivers for each pixel in the column.

FIG. 18 illustrates a block diagram of elements and features of theswitching circuit box.

FIG. 19 illustrates a simple control software algorithm implementing MLAon the peripheral switchbox.

FIG. 20 illustrates an initial prototype having 25 modules.

FIG. 21 illustrates the circuit schematic for the prototype power supplyand routing box.

FIG. 22 illustrates an initial 4 by 4 test module being driven by theoptimal MLA strategy.

FIG. 23 illustrates a 4×4 panel with activated cells.

FIG. 24 illustrates a prototype having 25 pixel sub-module.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While this invention is illustrated and described in a preferredembodiment, the invention may be produced in many differentconfigurations. There is depicted in the drawings, and will herein bedescribed in detail, a preferred embodiment of the invention, with theunderstanding that the present disclosure is to be considered as anexemplification of the principles of the invention and the associatedfunctional specifications for its construction and is not intended tolimit the invention to the embodiment illustrated. Those skilled in theart will envision many other possible variations within the scope of thepresent invention.

This invention focuses on applications to architecture, but the samedevice could be used in automobiles, marine vessels, aircrafts cameras,or any instrument that requires a selective and accurate regulation oflight through a transparent membrane.

FIGS. 1 a-d illustrates four main applications within architecture.

A) Large scale displays (see FIG. 1 a). For example, this could be usedfor advertisement, or art installations.

B) Small scale indoor displays (on room windows or other interiortransparent membranes, or suspended sheets) (see FIG. 1 b). For exampleshop windows could display information while allowing prospectivecustomers to look through the window.

C) Glass facades that selectively block incoming sunlight and daylightfor accurate environmental control (see FIG. 1 c). For example, existingtechnology that allows tracking the sun's movement could be used forblocking only direct sunlight and letting scattered light enter the roomfreely, allowing maximum natural lighting with minimal glair.

D) Modular visual partitioning of internal spaces could be achievedusing transparent membranes as dividers (see FIG. 1 d). For example onemay divide a room into smaller partitions so that to block visibilityfrom certain areas with the room. Such division will be dynamic asvisibility could later be partially or fully enabled.

The input for all of the above applications could come from severalsources. There could be a central control for an entire building thatwould provide image content to a large scale facade or an environmentalcontrol application. Images could be converted from a slowed down video,or any available digital image format, or they could come from a freehand drawing interface.

In parallel, there could be a room scale control that would allow usersto display their own graphics on the windows or partitions of the theirlocal space. Again, this could be a simple computer connection so thatit would be used like any screen, or it could have a free hand drawinginterface for displaying ideas and messages on the window,

Methods of Incorporating it into the Building

FIG. 2 illustrates the present invention's programmable facade 206 ofwindow modules 208 that is used on top of a building facade 210. Windowmodules are designed in panels so that they will tile together wheninstalled in a building similarly to standard glass panes, to form alarge display (see FIG. 2). The associated electrical connections ofeach panel will connect rails that will guide the electrical signal fromthe controlling units (e.g., from the control computer 202).

To simplify installation, the controlling units need not reside next tothe window. These units can be configured automatically to address ascreen of a scale that is defined by the number of window modules thatwere connected to the conducting rails during installation.

For display purposes during the night, it will be necessary toilluminate the exterior envelope that conveys the desired informationfrom inside the building (see FIG. 2).

Constructing the Window

There are several methods for constructing the window panel. Two mainconfigurations exist, the choice between which is dictated by thedesired method for controlling the pixels described below. The first isbuilt to facilitate individual wiring of one terminal of each pixel,with a common ground to all. In the second, pixel terminals areconnected together in common rows and columns.

To construct a window according to the first method, a transparent sheet(i.e. glass or plastic) is to be coated with a transparent conductor,where conductor distribution will determine the pixel shape and size.Each pixel is created by laying down a patch of transparent conductor: asquare shape usually makes best use of the area of the sheet, andcreated the smoothest image. Each patch should then be individuallywired with a transparent wire or a line of transparent conductor, andall wires from all patches should meet at one of the sheet's edges andconnect to a terminal. Another transparent sheet should be entirelycovered with transparent conductor and connect to a conducting wirewhich should in turn be connected to a terminal. A layer of suspendedparticles, electrochromic materials or liquid crystal is to be placed onthe transparent conductor that is on the first sheet, and the secondsheet should be pressed against the first, with the transparentconductor touching the suspended particles, electrochromic materials orliquid crystal. The two-sheet combination should then be laminated. Thewire terminals of the first sheet are to be connected to the computercontroller and the one from the second sheet to the ground.

To construct a window according to the second method, a transparentsheet (i.e. glass or plastic) is to be coated with a transparentconductor, where conductor distribution will determine the pixel shapeand size. Each pixel is created by laying down a horizontal strip oftransparent conductor, where the height of the strip defines the heightof a rectangular pixel. Each strip should then be individually wiredwith a transparent wire or a line of transparent conductor and all wiresfrom all patches should meet at one of the sheet's edges and connect toa terminal. Another transparent sheet should be coated with verticalstrips of transparent conductor, where the width of the strip definesthe width of a rectangular pixel. Each strip should then be individuallywired with a transparent wire or a line of transparent conductor and allwires from all patches should meet at one of the sheet's edges andconnect to a terminal. A layer of suspended particles, electrochromicmaterials or liquid crystal is to be placed on the transparent conductorthat is on the first sheet, and the second sheet should be pressedagainst the first so that the transparent conductor strips areperpendicular to each other, with the transparent conductor touching thesuspended particles, electrochromic materials or liquid crystal. Thetwo-sheet combination should then be laminated. The wire terminals ofboth sheets are to be connected to the computer controller.

In order to develop the desired application it is necessary to use amaterial that can change the opacity of a transparent sheet and beelectronically controlled. Several such materials exist, for exampleSuspended Particle Devices, Electrochromic Devices and Liquid Crystals.A test of these materials showed that, SPD is the best option for theintended product concept. When appropriately powered, SPD is advertisedas having the lowest power consumption, it is easier to produce, operateand scale. The desired application was developed using SPD, but it couldhave been done with other materials such as the ones mentioned above, orwith others that might be introduced in the future.

The Pixel Controller

First we should talk about the controller in an abstract manner, i.e. adevice that allows the appropriate distribution of voltage to pixelsacross the sheet and takes commands from a computer or microcomputer.The command to the computer could be input by a variety of means, suchas drawing boards, images, videos, sensors coupled with environmentaloptimization programs, and other time activation schemes.

The commercially available SPD film can be modeled as a capacitive load.The degree of transmittance (transparency) can be controlled by theamplitude of the AC voltage applied to the plates and the frequency atwhich this driving signal is applied.

First, the film needs to be driven with a low power but high voltage ACsignal. Specifications from the manufacturer indicate operatingamplitudes of 80 to 220 VAC and a frequency between 50 and 60 Hz. Thehigh amplitude will necessitate an adequate power supply and conversionsystem. The AC voltage requirement means in addition that if we are touse a matrix structure to drive the display, we need to be able to bothsource and sink current from each of the independent row and columndrivers.

The lower and upper bounds on the frequency of operation will also needto be considered, given the capacitive nature of SPD. At highfrequencies, the low pass behavior will reduce the voltage amplitudeappearing across the film. The low frequency bound on the other hand maydictate the feasibility of using multiplexed or refresh type techniquesto drive the eventual prototype or may limit the number of pixels thatcan be controlled at a time.

The fact that the film is a bipolar load may complicate matters evenmore when attempting to use a common-cathode, common anode matrixdisplay technique. Since induced current will be able to flow in bothdirections across the load, applying a voltage to one pixel in a matrixwill possible affect the opacity of other pixels, because of undesiredpaths to ground. This problem is referred to as crosstalk or ghosting,and may cause several untargeted pixels to become activated.

The desire to drive each pixel at different opacity levels and not justin binary mode creates the need for an amplitude control switchingphase, adding an extra burden to the total processing time per pixel.This feature is again important with regards to matrix or multiplexedtype driving techniques. Other issues may result from the actual panellayout and bus wiring.

The present invention provides, in one example, the ability to controlone hundred independent pixels, with the possibility of applying atleast 64 voltage levels to each of them (1 byte of information)—alimitation that is neither taught nor suggested by the prior art. Abasic implementation of the present invention is shown in FIG. 3.

The SPD pixel is used in a low pass filter configuration, placing it inseries with a resistor and the frequency response of the combined systemis obtained. A 30 Volt peak-to-peak sine wave of a controlled frequencyis applied across the resistor and pixel. The peak-to-peak pixel voltageis recorded as the output voltage and the gain is calculated as thequotient of output and applied voltages. FIG. 4 illustrates the decibel(dB) gain responses for the two SPD pixel sizes in series with a 100kresistor of 1% tolerance. The best fits to ideal first order frequencyresponse magnitudes are superimposed on the measured responses, as ameans to check for the first order nature of the tested systems Theranges of capacitance for the both small and large pixels are about0.7-0.9 and 1.4-1.8 nF respectively. This information can be used tostart shaping the design of a driving system or circuit.

The fact that there is an Equivalent Series Resistance (ESR) (fromwires, pins, transparent conductor, etc.) implies that only half of theenergy supplied to charge a capacitor actually ends up stored on it,regardless of the size of this finite resistance. Since the storedenergy is given by ½CV², the total required energy is then CV². Powerconsumption will depend on how often the capacitor voltage needs to becharged or discharged, that is, on the frequency of the waveform appliedto the pixel and so is given by equation (1) below:P _(pixels) =C×V ² ×f  (1)

The film manufacturer specifies a range of recommended driving signalswith amplitudes between 80-280 VRMS and frequencies between 50-1000 Hzand advertises a continuous power consumption of as low as 0.06 Wattsper square foot (˜0.65 W/m²). The matrix shown in table 1 below presentsthe estimated power consumption for each of the four combinations ofextreme driving signal voltages and frequencies, obtained using themeasured SPD pixel capacitances and the known pixel areas.

TABLE 1 Power consumption matrix for combinations of extreme drivingsignal voltage and frequencies. The tabulated values are in Watts/m.

In the best case scenario, the calculated power consumption agrees tosome level with the low values marketed by the supplier. For a frequencyof 50 Hz and an 80 VRMS amplitude, for instance, the estimatedconsumption is of 70 mW. It is important to note that in order for theSPD to remain attractive from the power consumption perspective eitherdriving frequency or voltage should be as low as possible. One wouldideally want to use the film under the conditions of the top left cornerof the matrix above. We come back to this issue later, when determiningthe optimal driving strategies for the system.

The degree of transparency is related to the strength and the durationof the alternating electric field between the two conducting plates andhence to the applied voltage. For purposes of controlling the opacity ofa pixel, the notion of a linear transmittance or transparency range mustbe incorporated into the design, even though this variable is anon-linear function of applied voltage. The relation is linearized usinga transformation function. That is, if specific percentage transmittanceis required, the value is fed into this transformation function and thevoltage which is expected to produce said transmittance is calculated.%{circumflex over (T)}=40 tan h(0.05(VRMS−20))+42  (2)The linearizing function (2) maps the desired transparency to thenecessary voltage amplitude and it corresponds to the inverse of thebest fit function:

$\begin{matrix}{{VRMS}_{required} = {{20\left( {{{arc}\;{\tanh\left( \frac{\%\mspace{14mu} T}{40} \right)}} - 42} \right)} + 20}} & (3)\end{matrix}$This transformation will be added to the control software, and willrepresent the last step before downloading commands to the actualswitching system board.Description of the Different Ways to Drive an SPD Pixel;

Given the electro-optical characteristics of the SPD film, there areseveral possible driving techniques that could facilitate theappropriate signal to drive the pixels. These are reviewed along withseveral possible switching topologies, and specifically that of a dualMOSFET totem pole.

The structure and associated behavior of the SPD film necessitate analternating driving voltage for the control of the film's transparency.The first question to be asked therefore is how to produce an effectivehigh voltage AC signal across the pixel. Fine control of thetransparency levels is also part of the design concept, and it should bebrought about by manipulating features of the applied HV signal. Inaddition the solution to the previous two problems must be extended toan array of pixels. What follows is an outline of feasible alternativesto tackle the stated problems.

When power dissipation and regulation are not pressing issues, the ACmains are a reasonably reliable source for a high voltage waveform. At110-120 VRMS in the U.S (˜155-170 V amplitude), the mains provide an RMSvoltage in the saturated region of the voltage vs. transparency relationpresented in the previous section, at a marginal component overhead andwith no need for voltage amplification. The major disadvantage arisesfrom the lack of flexibility, since under this topology the drivefrequency cannot be controlled.

Alternatively, a digital circuit could be used to produce a zero offsetwave of more easily controllable parameters. A Digital to Analog(DAC)-based solution or frequency-mediated sine wave generator ICs couldbe used for such task (see publication to Philip Krein titled “Elementsof Power Electronics”). In either case, these solutions would entail anamplification stage to be able to reach the high voltages needed foroperating the SPD pixels at maximum transparency levels.

Another option to recreate AC drive signals across pixels is to useDC-AC conversion by effectively sampling from HV DC power rails, andalternating the polarity of the connections to the pixel terminals. TheHV levels could be obtained readily by rectifying the mains AC signals.This solution stands out for its versatility, as more of the signalparameters can be varied and their effects analyzed.

The load characterization clearly depicts the capacitive behavior of theSPD pixel. The effective voltage across a capacitor in any form of RCcircuit depends on a fixed DC gain and the frequency of the wave beinginput to the system. Being transparency a function of RMS pixel voltage,varying gain and/or drive frequency are the most straightforwardstrategies for transparency control.

Clearly, by changing the effective amplitude of the input signal, theelectric field strength and hence the transparency level are directlycontrolled. However, given the inherent series resistance (be it fromwires, the pixel conductive layers or from an external resistor), thelow-pass nature of the pixel-resistance aggregate system yields yetanother method to control the transparency, taking advantage primarilyof the attenuation of pixel/capacitor voltages at high frequencies.

For either the direct AC wave drive or the sampled DC drive directamplitude control would require a variable gain or amplification stage.Under the HV AC alternative direct amplitude control requires a variablelow voltage AC wave generator followed by a HV amplification stage,possibly a transformer. The amplitude of the low voltage AC signal, andconsequently of the amplified output wave, can be easily and digitallycontrolled using a digital to analog converter or a digitalpotentiometer.

The major issue is clearly the amplification stage. A survey ofcomponents in the HV niche yielded a line of high voltage operationalamplifiers by APEX semiconductors, the major disadvantage being theextremely elevated cost, barring this alternative from the space offeasible solutions.

High-voltage adjustable regulators could also be an interesting option.The Texas Instruments TL783 (see Texas Instruments Datasheet of TL783),with an adjustable output range of 2-125V is perhaps a suitable choice.Although the in-depth discussion is deferred for later, the maximumvoltage constraint will probably limit its use for strobed ormultiplexed systems. That is, if one single HV rail is to be used, thedriving strategy will necessitate some form of strobing or multiplexing.When this is the case, in order to apply an effective voltage of over100 VRMS, the applied samples or pulses must be high enough to averageup the periods in which other pixels are being serviced. Having themaximum value capped at 125 V may represent a problem in this context.

It is also possible to resort to high voltage transistors and develop anapplication-specific HV variable supply, which would certainly relax theminimum limit on the HV level. Several basic design alternatives arebriefly introduced in the publication to Paul Horowitz titled “The Artof Electronics.” The design of such supply, essentially an operationalamplifier, would require feedback analysis to produce a fast slew-rateand response and be able to switch the amplification level quicklybetween when pixels are being serviced. The determining step would bethe characterization of the gate-signal to output-signal frequencyresponse of the switching element, to be able to close a control looparound the level converting stage. Although also applicable to the fixedDC rail alternative, the overall cost, now in terms of time, deems thisoption equally undesirable.

In theory, however, it is also possible to have a series of discrete andfixed DC voltage rails, switching the desired voltage onto the requiredpixel. This option would only be practical up to a certain number ofdiscrete voltage levels, a greater range demanding considerablecomponent overhead—basically one switch per voltage level. Thisadditional complexity would add to that of the switching or drivingdevices. What is more, given the wide range of voltages that producedistinct transparency, more than just a few discrete tones or voltagesettings will be necessary. Since the response appears monotonic atleast before the evident plateau, to maximize the pixel's informationcontent the control levels should be as many as possible.

The main advantage of resorting to a frequency dependent controlmechanism is that only a single non-adjustable voltage level isrequired. This technique takes advantage of the low-pass behavior of thewire-pixel system. Since both the wire resistance and the measured andverified effective capacitance of the pixel are insignificant, however,the RC time constant of the system will be extremely low, giving it anenormous −3 dB bandwidth—with C at 2 nF and a wire resistance of aboutless than 1 Ohm this number is at around 5 GHz! Consequently amplitudecontrol would have to be brought about using signals far into the HFspectrum unless a large resistance is added to the driver output.Although possible, such high frequency switching may be hard toimplement without taking the design complexity to another level.

Most dimmer-switches resort to phase control to regulate the amount ofcurrent being sourced into a load. A bi-directional pass element,usually a thyristor and more precisely a TRIAC, blocks the current pathat a certain phase angle (time from the zero crossing) of the AC wave.The longer the blocked portion of the AC wave, the lower the apparentintensity of the bulb. In the case of a resistive load (bulb), acrosswhich voltage changes instantly, visual perception represents thelow-pass filtering stage.

The phase angle is usually adjusted using a potentiometer between thesignal (mains) line and the TRIAC gate (current drive). Digital Controlcould be incorporated by having the high voltage AC wave, feeding aclipped version through a zero crossing detector circuit and use aninterrupt initiated process to control the amount of time to leave theTRIAC pass element in the on state. The driving of the TRIAC could beperformed with high current drivers.

The DC analog of phase control is Pulse Width Modulation (PWM), whichessentially uses constant voltage pulses of varying width to control orvary an effective applied voltage. If the on-off switching or pulsing isdone fast enough, the net effect is an average voltage given by theproduct of the pulse voltage and the duty cycle. In this context, pulsewidth modulation (PWM) effectively provides a means to control the RMSvoltage and indirectly the transparency of the SPD pixel.

PWM depends on the accurate manipulation of the switching waveform (e.g.when varying pulse widths precisely), this usually not being a problemat the logic level. To setup a PWM strategy at the voltage levelsrequired to drive the SPD film, on the other hand, the driver must becomposed of fast switching components. This requirement points in thedirection of the MOSFET. The main advantage of such a switching solutionis the minimal design overhead in terms of power electronics, sincethere is no need for the pixel drivers to have complicated high-voltageadjustable supplies or bulky transformer stages (except to providemetallic isolation). PWM just requires a stable HV rail, easilyobtainable from the isolation and rectification of the AC mains waveformand an appropriate gating component to source current from it.

The many alternatives discussed so far tackle the problem of driving anAC wave onto the SPD pixel. The number of feasible solutions changeswhen this type of drive is extended to a matrix arrangement of suchpixels, as would be the case with a display. Although TRIACS might beadequate for strobing successive phases of a HVAC wave, the displayconstraint still pushes towards a MOSFET-based solution on the groundsof component size, high switching speeds and practically full control ofthe effective cross-pixel applied wave (see publication to Paul Scherztitled “Practical Electronics for Inventors”).

High-voltage rails and switching elements is probably the optimal designto dirve the pixel in terms of flexibility. With the MOSFET solution,frequency, pulse width and also polarity can be digitally controlled, auseful capability to have in the early test stages of a prototype.

Displays usually resort to some form of multiplexed or strobed drivingtechnique, where the switching times are one of the main constraints onthe size of the display. This is particularly the case when the voltagesto be switched are considerably high due to the need for high slewrates.

The main idea behind a level converter system is to have two passelements, to source and sink current to and from a load. A MOSFEThalf-bridge configuration does precisely that. When arranged in a totempole or half-bridge topology, combinations of N and P channel MOSFETScan be used to source and sink current from positive and negative HVrails, to provide a path to ground, or to appear as a hi-impedanceoutput. The conversion from digital logic to over 100 Volts, moreprecisely the gating of the HV power MOSFET, calls for special driveconsiderations.

In a totem pole setting, two MOSFETS are connected in series and used aseither source or sink device with the appropriate control logic. If twoN-Channel type devices are used in a source-drain alignment, the topMOSFET's source terminal actually floats on the half-bridge output. Thedevice cannot be gated with a voltage referenced to ground, since theconductive state is reached when its gate-to-source voltage reaches athreshold.

A thorough discussion on the issue of high-side or floating drive ispresented in the publication to International Rectifier titled “An-937Gate Drive Characteristics and Requirements for HEXFET Power MOSFETS”and several plausible design routes are recommended. Among these, andpresented here for its simplicity, is the bootstrap capacitor method forhigh and low side driving of a dual N-channel MOSFET half-bridge.

FIG. 5 illustrates a basic dual N-channel half-bridge driver using abootstrap method for high-side gating.

The high side gating occurs when capacitor C1 charges through the fastrecovery D1N4148 diode as the low side power driver sinks current fromthe output node. When the low side power MOSFET stops conducting, thecapacitor is effectively holding an above threshold gate to sourcevoltage that floats on the output, just like the source of N-ChannelMOSFET. In order to source a high voltage, therefore, the high sidedriver input signal must be low and the low-side input must be high. Themaximum pulse width is dictated by how well the bootstrap capacitormaintains its voltage.

Additionally, one could also just choose to use a common drain CMOShalf-bridge, with the advantage of being able to reference the high-sidegating of the PMOS to the HV rail and the low-side NMOS to the oppositesupply. The basic driver topology for this alternative is illustrated inFIG. 6.

In this driver circuit, the high-side P-Channel MOSFETS are gated by thevoltage temporarily appearing in the source-gate resistor as the voltageat the base of the RC circuit changes. Assume, for instance, that thesystem starts in steady state. The gate capacitor is charged to +HV or150 Vdc in this case. When the MOSFET driver's output goes high, thegate will actually be at a higher level than the PMOS source until thecapacitor discharges. When the MOSFET driver output goes low again, thecapacitor now charged to less than +HV will actually trigger the topPMOS. The maximum width of the applied pulse is controlled by playingwith the RC time constant.

Perhaps the only difference between the two presented driver designs isthe ability of the RC gated drivers to supply both +HV and −HV pulsing.Again in the interest of adaptability, versatility and simplicity, thisdriver is selected for the support test platform.

An objective was to build a system that grants enough freedom and powerto vary the many possible features of the signal applied to an SPDpixel. The basic discrete element system is built around the half-bridgeMOSFET drivers described earlier. Each N and P channel CMOS pair has anassociated level conversion stage, allowing the gating of the high-sideMOSFET and a power driving chip interfacing it to the digital logiccontrol circuitry. The operation of the control logic, the timing andprogram flow are controlled in software using the micro-controller unit.

The selected microprocessor, the ATTIny26, is part of the RISCarchitecture microcontroller family by ATMEL. Among the main on-chipfeatures are multiple function I/O ports, an on-board ADC, timers and anexternal interrupt. The chip offers 2 KByte flash memory space forprogram download, 128 Bytes of EEPROM and can be clocked internally torun at 16 MHz with no additional components.

The main operational functionality and the first level of designabstraction are brought about by a simple C function calledupdate_latch. The function takes in a COLUMN, a ROW, a PHASE flag and aPULSE WIDTH parameter and proceeds to turn on the drivers correspondingto the pixel at the row and column intersection accordingly.

This Sections Reviews Ways to Apply Variable Voltage AC Signal to thePixels:

In general driving circuits for display technologies are classified intotwo broad categories, direct driving and multiplexing. The loadcharacteristic dictates which category the display system will fallinto.

Under direct drive, each pixel's signal electrode has its own circuitdriver and is electrified independently of the other pixels, but at thesame time. The anode or ground electrodes are usually tied together.There is therefore a continuous application of power and since noperiodical refreshing is required the system is also referred to asstatic driving.

This addressing method is perhaps both the simplest and most robust. Forinstance, a failure in one driver element would only affect a singlepixel, whereas in the case of matrix or multiplexed addressing, it wouldaffect a whole column, row or concurrent group of pixels. In addition aclose connection between pixel and driver allows for easy implementationof a compensation or feedback system to stabilize the pixel outputagainst component performance loss (see thesis by Eko Lisuwandisubmitted to M.I.T. in 2003 titled “Feedback Circuit for Organic LEDActive-Matrix Display Drivers”).

In spite of these advantages however, the lack of scalability limitsdirect addressing to very low information content displays. For a smallarray of pixel, the easiest way to switch a variable high voltage signalto each SPD pixel would be to assign each a high-voltage source and acontrol or switching totem-pole driving element. For a 100 pixel system,however, this technique would translate into more than two hundreddiscrete components, a problem both in terms of cost and bulkiness. If nis the number of pixels under static driving, the complexity, componentuse and cost of the system grow linearly with n.

For purposes of the intended application, continuous application ofsignals to each pixel is not efficient in terms of power consumption aswell as controller size. On top of the systematic desire for low powersystems lies the issue of the large-scale feature of this displayapplication. The main rationale for the use of SPDs in this applicationis their trivial power requirements. If their use were to be extended tovery large surfaces one would want to keep the consumption as low aspossible for such an application to remain attractive.

Although it would definitely minimize both control lines and drivers,the main reason to resort to some form of strobing is therefore toreduce power consumption. By way of illustration, take a 100 pixeldisplay, each pixel being one of the 9 cm a side squares characterizedpreviously. The theoretical power consumed by one such pixel when a Vvolt 0-offset square wave is applied to it is of ˜CV². When using directdrive, all 100 pixels are using said amount of power. If, on the otherhand, the driving circuit were charging one pixel at any one time thenthe required power would be just 1% of the previous total. In idealterms, single-pixel strobing could therefore allow one to drive 10,000pixels with the same power consumption used to direct drive 100 pixels.As a matter of fact, the actual power consumption would have to includethe gate and output capacitances of the driving elements and the powerusage of control circuits, but the figure would still be much lower thanunder concurrent drive.

A Discussion of Multiplexed Driving Strategies; Strobing an AC Wave Ontoan SPD Pixel:

The alternative of multiplexed drive stands as possible solution to thescalability and power consumption considerations of this particularapplication, at the expense of some robustness and operationalsimplicity. The display is set up as a matrix structure with commonanode rows and common cathode columns each assigned an independentgating element. In square displays, the complexity and component use nowgrow as the root of the total number of pixels.

The benefits of multiplexed drive in terms of the amount of signals ordrivers that must be controlled are evident. For a 100 pixel display, wewould be talking about a 90 gating component difference with respect toAC direct drive. The implementation of multiplexed drive requires,nonetheless, thorough consideration of the load responsecharacteristics. The need for fast switching necessitates faster pixelresponses as well as an extremely precise switching control.

In traditional multiplexed drive, gating elements are locatedperipherally, opening up a path from signal (source) to ground (sink)across a component connecting a column and a row bus. In active matrixtechnology however, signals control a gating element, typically MOSFETswitches, which are connected in series with the load. One set of linescarries the voltage signal to be applied to the pixel device, and theother set controls the on-pixel gating. Active matrix driving thereforestill requires one gating component per pixel, but it makes use ofmultiplexed control and power signals to reach each of them. That is,there are savings with regards to control lines but not to the requirednumber of drivers.

SPD film has no state memory and consequently has to experience acontinuous voltage waveform of specific amplitude across its terminalsin order to remain transparent. Its response, specifically its turn-offtime, is therefore particularly important in establishing whether arefresh system could be used to drive it. It is seen that althoughsteady state transparency levels do follow the amplitude of the appliedvoltage, the voltage rise times and decay times are much faster thanthose of the transparency variable.

When dealing with SPDs in which organic particles are suspended betweentwo conductive parallel plates there exists a time-constant related tothe changes in the cross terminal voltage. A second time constant isassociated to how fast random motion disorganizes the particles in thesuspension when there are changes in the AC-field, and it is quitelikely related to variables such as the suspension viscosity, or anyother variable affecting the diffusivity of the organic particles.

In the context of displays, and precisely of strobing or pulsingstrategies, the slow dynamic behavior of this transparency variable isof great advantage. It implies that once the transparency is obtained,it takes but a simple update pulse at a certain cuttoff frequency tomaintain that state. In this qualitative analysis the scaling of thephotodiode output is not an issue. Independent of the vertical scaling,the time evolution provides a great insight into the speed of thetransparency decay times.

The rise times in transparency are faster than the fall times. Whereasin the rising transparency stage an electric field is forcing thealignment, on the falling transition only random motion causes thedisordering of particles. The rising time constant is of about 500-600ms and the decay time constant of about 3 seconds.

As mentioned before, the slow decay times may be advantageous from themultiplexed drive standpoint, since they provide some leeway on therequired pixel update frequency. Transparency does start changingimmediately after the drive voltage is removed. Intuitively, the lack ofsignal just after the oscillatory pulse stops implies that the averagevoltage applied per unit of time is reducing slowly with time.Transparency follows the reduced average voltage and drops accordingly.

A key issue here is to make sure that the servicing frequency togetherwith the applied signal voltage, give an average applied voltage thatwill drive the transparency level to the required level in the firstplace. The transparency variable is still a function of steady state oraverage voltage after all and, although the decay will be slow, thepixel will lose transparency if the average voltage applied to itdecreases.

The control system will allocate each pixel in the display a specificservice time. During this time, the signal properties will have to beadjusted to produce the effective AC drive and the required transparencylevels. A series of alternative approaches to tackling these two issuesare exemplified in the next section.

Matrix addressing guarantees that all pixels are independently reachableeither with fewer components (passive) or just fewer control signals(active), but a more complex system needs to be built around it to beable to display an image that requires several pixels to exhibitdifferent opacity levels concurrently.

In light of the AC signal operation of SPD, successive services to eachpixel would have to be of opposite polarity, a requirement that adds tothe complexity of the problem. The following discussion presents anoverview of different strategies to multiplex AC signals onto an SPDpixel using the MOSFET based test platform. Each driver can providethree states, HI, LO or high-Z.

This technique handles strobes by phases, first handling the positivephases for all pixels and then their negative phases. During thenon-servicing time, the drivers are set to high-impedance or floatingstate.

Since the pixel is left floating during the non-serviced period, itremains charged. Because even a small pulse charges the output voltageto the max rail value, the effective amplitude cannot be controlled byvarying the pulse width during the service time. The alternative is toground both pixel terminals after the required pulse width has beenapplied, discharging the capacitor voltage before floating its terminalsand causing the desired effect on the transparency. Note that the pixeltransparency, as evident from the photo-detector output, remains at thesame level as before which confirms there is not need to leave thecross-pixel voltage charged between services.

The service time for this strategy is constrained by the minimumrequired driving frequency for SPD film of 50 Hz. At 50 Hz, the halfperiod is of 10 ms. For a 100 pixel display, each pixel would beassigned only a 100 μs servicing time. The issue here is, as wasexplained earlier, that the required operating voltage will have to beadjusted to make sure that the effective RMS or average voltage canactually cause the maximum transparency level.

A third alternative is the application of a bipolar pulse. FIGS. 7 a-billustrate bi-polar pulsing of transparency variable. Under thisstrategy, the pixel is completely serviced during its allotted time andit in theory gives the transparency variable a pulse-like kick. As longas the frequency of the +/− pulse is within the specified SPD limits,the servicing rate must be constrained by the perceptual fusionfrequency (˜10-20 Hz) and the actual RMS average voltage the pixel sees.If we would like to keep the HV rail at a reasonable level (i.e. below˜300 VRMS maximum rating of the SPD), the maximum duty cycle that couldbe used to attain the full transparency (reached with 60 VRMS) would beoff about 60/300 or about 20%. That is, only five pixels would be ableto be serviced before restarting.

As a final note, the equivalent plot is presented for continuous driveat the same corresponding frequency. FIGS. 8 a-b illustrate transparencylevel with continuous drive under an equivalent voltage level andfrequency. The photo-detector voltage is at the same level as in theprevious cases, showing that a pulsing strategy can produce the sametransparency level as continuous drive at this particular refreshfrequency range of about 100 Hz.

There are several design strategies to reduce the voltage required forstrobing a display. The first is to resort to a modular design. Insteadof using a 100 pixel unit, for instance, four 25 pixel modules could beused and interconnected. The strobing would now go only through 25pixels on every refresh, increasing the effective signal's duty cycle.In light of the safe 20% on-off duty cycle calculated earlier, strobingacross 25 pixels may also limit the maximum transparency. Fortunately,considerable improvements can also be attained by resorting to amulti-line strobing technique within each of the modules.

Multi-Line Addressing lies between the extremes of direct drive andsingle-pixel strobing. Instead of multiplexing over every pixel, acolumn of the display array is handled at any one time. At the start ofthe column service, the untargeted columns drivers are set to a Hi-Z orfloating state. The row drivers are set or cleared depending on whichpixel in the column needs to be activated. Amplitude control is handledby leaving the corresponding row driver on for whatever duty cycle isassociated to the desired transparency. After one phase is completed,the driven column reverses polarity and the rows are activated asrequired to apply the reverse polarity voltage for the specified dutycycle.

FIG. 9( a) above shows the required COL and ROW drive waveforms toproduce different transparency levels in a 2 by 2 sample pixel arrayusing MLA. The signal P1 applied to pixel 1 is controlled by the on/offtime of the row one (R1) waveform when column one (C1) is beingasserted. The proper functioning of the MLA driving technique requirescolumn drives to be able to produce a Hi-Z state when inactive. Thiswould prevent the constantly running rows waveforms form affectingpixels in untargeted columns.

FIG. 9 b illustrates an ideal waveform applied to P1. FIG. 9 cillustrates an ideal waveform applied to P2.

We choose multi-row (i.e. one column at a time) by way of illustrationbut the truth is one could go as far as full scale-direct matrixaddressing if power dissipation or consumption levels allowed for it.The decision to use multi-line addressing resulted from trying to find amidpoint between power consumption constrain (high bound) and thetransparency response minimum RMS input voltage (low bound).

The driving strategies reviewed thus far all require 3-state drivers atleast for the column lines, as is the case with MLA. The tested platformprovided this feature and this should be a requirement of any drivingintegrated circuit to be used for a final prototype.

In implementing the multi-line driving strategy in a pixel array, apassive matrix wiring topology would represent the easiest way to strobesignals onto it. Given that SPD film is a bidirectional conductiveelement, however, this type of addressing is very sensitive to theproblem of crosstalk, that is, to the activation of unwanted pixels dueto the common-cathode common anode wiring. This problem is referred toas cross-talk or ghosting, and may cause several undesired pixels tobecome activated (see publication to Ohta et al. titled “ElectronicDisplay Devices”).

Consider a 2 by 2 matrix. To electrify the top-right pixel a voltagemust be applied by asserting row 1 and column 1. If the two other linesare left floating the desired pixel sees +V across it, while untargetedpixels 2, 3 and 4 split up the +V volts across them and this is theresult of pixel P4 being able to conduct in both directions.

To minimize crosstalk the unused rows and columns are sometimes groundedand the signal on the target pixel is applied using bipolar drivers (asshown in FIG. 10( a)). As seen in FIGS. 10 a-b, when column one is setto +V and row 1 to −V, P1 sees 2V across it, P2 sees −V, P3 sees +V andP4 experiences no crosstalk. Ghosting occurs in both cases, althoughaffecting a different set of non-targeted pixels. If the driven load hada response threshold set anywhere below V/3 or V/2 volts, any of the twoalternatives proposed above would work, since untargeted pixels would beable to see up to V/3 or V/2 volts across them and still not be active.

A review of the cross-pixel signals provides a clearer understanding ofwhat happens. The voltage across the untargeted top-row pixels is abouthalf the one applied to the targeted pixel. FIG. 11 illustratescrosstalk during multi-line addressing, occurring when row pixels areassigned different pulse widths. FIGS. 12 a-b illustrate multi-lineaddressing and cross-talk. Under multi-line addressing cross-talk occurswhenever two pixels in the active column are being applied differentpulse widths. When the pixel in row one is still ON but the pixel in rowtwo is grounded, current can flow from row 1 (still on) to row 2(grounded) across pixels 3 and 4.

The two untargeted pixels thus split the voltage between them, asclearly evident in the next plots, which show the target and non-targetpixel waveforms at the maximum difference in pulse width between the row1 and row 2 signals. As expected, during each phase (half-period) thetwo pixels on the unasserted column split the applied voltage, causingeach to see half the amplitude.

Crosstalk results from SPD film not having an activation threshold. Ifthis were the case, passive matrix addressing would still be an option.This motivates an attempt to tackle the problem by looking for ways toartificially recreate a threshold on the response of the SPD pixel.Ideally, the I-V characteristic response is as shown in FIG. 13.

Under such a threshold, the pixel load would conduct bi-directionallybut only when a high enough voltage is applied. If we use HV Pulse WidthModulation (PWM) for amplitude control this threshold would just have tobe below the HV rail being sampled. Passive matrix wiring would be usedand the activation cutoffs would block the half voltages that wouldotherwise affect untargeted pixels. FIG. 14 illustrates a diode-basedsolution to impose the desired V-I response curve. A pair of Zenerdiodes in series with the load conduction path and connected in opposingpolarities could provide a fair approximation. This arrangement wouldactually be reproducing the effect of a TRIAC bi-directional switch.

Instead of imposing HV threshold, the crosstalk problem might also betackled by eliminating the bidirectional conduction requirement of theSPD drive signal and utilizing blocking diodes. This strategy wouldpreclude the use of a bipolar AC wave to drive the pixel and should intheory make no difference since, from the pixel perspective, all thatmatters for the alignment-induced transparency is the presence of analternating E-field.

The response of the Zeners and diodes in general is not ideal. The V-Icharacteristics are more along the lines of the dotted curve in FIG. 13,with a small leakage current existing well before the diode forward orZener reverse voltages are reached. Given the extremely low RC timeconstant of the pixel-wire system, even such small leakage currentswould suffice to charge the pixel to noticeable voltages even when thediodes have not been activated (i.e. when it is not supposed to charge).The artificial threshold solutions would work in resistive loads (onwhich trivial currents also have trivial effects) but not on smallcapacitive loads as the SPD pixel.

Another alternative approach to incorporate a threshold might resort tofrequency modulation alternatives. Although the capacitance of thepixels is very small, the ultimate variable of interest has a muchlarger time constant. High frequency drive could be used againstcrosstalk, taking advantage of the roll-off (i.e. threshold) in thelow-pass filter frequency response. The targeted pixel could be drivenby an intersection of a constant HV level and a high frequency wavecausing the net voltage to be at a highly attenuated frequency.

If the anti-crosstalk strategies discussed earlier were to beimplemented, the current SPD film with which this research was carriedout would require there to be a discrete component in series with everypixel. In an attempt to minimize the sacrificed pixel area each of thepixels would have to be reached independently and the blocking orthreshold-imposing components would have to be wired in series in aremote location away from the actual panels.

That is, the artificial addition of a threshold would require the panelof pixels to be directly driven or wired anyway. This being the case, wecould add full-control of the threshold characteristics by placing a HVdriver at one end of every pixel. Given our AC drive constraint, a totempole driving common cathode would have to be used at the other end ofthe pixels. This topology would actually be implementing an AC form ofactive addressing, since one side of the pixels are reached by rows andthe other one by columns, each of the column pixels having anindependent driver.

With regards to the required pixel wiring, MLA could be thought of asimplementing a strobing technique in a directly addressed panel. Thisapproach reconciles the threshold advantages of direct drive—addingcontrol over the threshold via a switch—with the power consumptionsavings associated to a pulsed or time-multiplexed activation. Permittedby the low information content of our basic controllable model,direct-driven MLA was shown to work perfectly on a test with the 2 by 2pixel array.

FIGS. 15 a-b illustrate pixel brightness under direct addressing and MLAstrobing. FIG. 15 a illustrates a setup comprising four pixels, 1502,1504, 1506, and 1508, formed on a single transparent membrane 1510,wherein only targeted pixels (i.e., pixels 1502, 1504, and 1506) are on.FIG. 15 a-b shows that there to be absolutely no cross-talk when usingMLA under direct drive. FIG. 15 b illustrates the results confirmedafter digital image analysis. The waveforms measured across the targetand not-target pixels under the current solution are illustrated inFIGS. 16 a-b.

A prototype, introducing key design and engineering elements, could bedrawn from in future versions of this system. In an attempt to refinethe design and reduce the peripheral control box sizes, the prototyperesorts to off-the-shelf or commercially available High-Voltageintegrated circuits and, with a clear set of design considerations,works around the constraints imposed by their operational ratings.

Supertex products were chosen for the system due to their ability tooperate at voltages of up to 250 V. For a 25 pixel module usingmulti-line addressing, for instance, the voltage needs to be cranked upin order to make the pulse-triggered transparency level equal to thatobtained from continuous drive.

In one example, the final prototype consists of four interconnected25-pixel modules. These modules are driven using a multi-line addressingstrategy and, given the low pixel resolution per module, anactive-matrix wiring topology is chosen to make sure cross-talk isavoided. FIG. 17 illustrates a 2 by 2 pixel representation of the wiringand driving scheme applied to the basic window module. Note that eachcolumn driver consists actually of drivers for each of the pixels. Theyall share a common activation signal but there is common source driveonly along the row or horizontal direction.

Effectively still in a matrix topology, this design takes advantage ofthe multiplexed or pulsed drive associated to matrix structures and atthe same time uses a pseudo-direct drive wiring methodology.

The implementation of MLA requires column drivers capable of sourcingand sinking current, as well as being at high impedance (Hi-Z) state.The best fit is a combination of the HV7022 and the HV514. Both ICs areserial to HV parallel converters, the former controlling 343-statedrivers and the latter driving only 8 HV lines to either HV or ground.The operation of these devices is based around a low-voltage shiftregister into which data can be clocked, and a latch line that causesthe values in the shift register to act upon the high voltage outputs.

Each chip has a distinct mechanism to control the state of its HVdrivers. For purposes of this design, the two important control lines inthe HV7022 are the polarity (POL) and the output enable (OE). With othercontrol lines grounded, the state of the output driver is dictated bythe POL, OE line and the shift register (SR) state as dictated by table2 below.

TABLE 2 Truth table for the control of the HV7022 outputs [Supertex DataSheet]. SR POL OE OUT H H H H L H H Hi-Z H L H L L H H Hi-Z X X L All

The case of the HV 514 is straightforward. The latch enable (LE) linemust be low when data is being clocked into the device. When LE is set,the serial register contents are amplified and presented in atransparent manner at the outputs. The only minor issue with regards tothe HV7022 chip is the lack of a proper latch enable line. Theequivalent control is an output enable (OE) line. When OE is low alloutputs are in Hi-Z state so every time a data packet is updated thecolumns drivers will be floating.

The main feature of serial registers as those used to load data ontothese HV drivers is that they have both Data In and Data Out pins. Thatis, if several such devices are daisy chained (DO→DI and so on . . . )one can eventually think of the system as a giant shift register. Thecontrolling of the output states would in this case be dictated by theserial packet to be clocked into the devices. The switching system isdesigned with this idea in mind.

The basic features and capabilities of the switchboard system arepresented in the block diagram in FIG. 18.

The board contains an MCU for interfacing to a higher level controlsystem and to perform the low-level timing and routing. The serialtransfer chain is composed of the HC166 parallel to serial converter,the HV7022 column drivers and the 514 row driver. The serial packetconsists of 6 bytes containing the states of all 25 column drivers, 5row drivers and a polarity control variable. The position of thesecontrol bytes within the serial packet is indicated in table 3.

FIG. 19 illustrates a simple control software algorithm implementing MLAon peripheral switchbox. The flow control program keeps a track of thefollowing states: COLUMN, ROW, PHASE and PERIOD. On every periodincrement the program checks the current value against the correspondingvalue in a 5 by 5 byte array, essentially the image. The byte packetwill then be updated as required and will then be clocked out onto thesystem.

The packet is composed of 6 bytes, with bits within the packet assignedto specific drivers or functions along the aggregate shift register. Thestate of the program, dictated by the current COL, ROW, PHASE and PERIODvalue, and the image array values control whether the byte array must beupdated before each download. The multi-line switching algorithm ishandled by the MCU and the high-level flow-chart implementation ispresented in FIG. 19.

The serial packet is sent out in byte batches with the use of the 74HC166 parallel to serial converter. The MCU presents the byte on PORTA andthen latches it onto the converter. Immediately after, 8 pulses, (16edges) are sent out to clock all the daisy chained components and thebyte shifts into the first 8 slots of the aggregate register. Note thatthe connection from PORTA to the parallel input port of the convertershould be inverted to make sure that the least significant bit of theleast significant byte in the serial packet is the first to be clockedin (the one that will get all the way to DO of the HV514 chip). Theprocess is repeated for the remaining 5 bytes of the packet.

When the desired linear transparency is calculated, as a percentage oftotal transparency, this value is transformed into the required PWM dutycycle that will produce the requested transmittance. This value is thendownloaded onto the switchboard. The linearization process is carriedout on the computer side. Two methods are feasible, one having a lookuptable of previously calculated values and the second one calling theapproximate conversion function obtained previously on every download.Either of these could be used depending on any constraints on programlength vs. running speed.

Given the size requirements, a small microprocessor is used and thisputs pressure on the availability of control lines. The required controllines are the OE/LE, the CLK, the Parallel Load signal for the HC166parallel to serial converter and the POLARITY line for the HV7022. Afterthe requirements for communication and programming link, only three ofthe PORT B lines are available.

Instead of adding extra components, the control over the extra line isincorporated as a signal bit within the communication/download protocol.By making this bit the first important bit position in the serial datastream, it will always end up in the data out pin of the HV514, the lastchip on the serial chain. The DO pin could be used to drive any controlsignal. In this case, the POLARITY state of the HV7022, the control linedictating whether the outputs will transition between H or L and Hi-Z,is tied to the DO pin. The stream of data must be perfectly coordinatedsuch that at the time the OE and LE signals are asserted, the 514 DOline will be presenting the corresponding polarity value. The value ofthe POLARITY bit in the serial packet is dictated by the PHASE state inthe software.

The table provided below (table 3) shows the serial register and bytepacket protocol.

TABLE 3 Serial register and byte packet protocol.

The driver integrated circuits have maximum power dissipation ratingsthat constrain the number of drivers that can be set at any one time.For the HV7022 the absolute maximum rating is of 1200 mW. The powerconsumption calculation includes two main components. On one hand is thepower required to gate the high-voltage CMOS drivers. The HV7022datasheet indicates that a single high-voltage output draws 4 mA fromthe supply rail.

The MLA drive technique requires the concurrent driving of five HVoutputs, each of which is activated at most 20% of the time. Theremaining 80% of the time this group of five drivers is in Hi-Z state,waiting for the assertion of the other four columns. The powerconsumption to gate any set of five drivers is therefore given by:P _(drive) =V×I×n _(drivers)×DutyCycle=150*0.004*5*0.2=600 mW  (4)

On top off the 0.6 W needed to drive the HV outputs is the extra powerdissipated when charging the five SPD pixels that could be activatedsimultaneously. For an operating voltage of 150V and a pixel capacitanceof ˜1.8 nF, the required power amounts to:P _(pixels) =C×V ²×DutyCycle×n _(pixels)=1.8×10⁻⁹×150²×0.2×5=0.4 mW  (5)

The total power consumption for this MLA implementation hence comes outto ˜600 mW, most of which is used by the driving circuitry. This valueis below the recommended maximum absolute rating of the HV driver IC.

The hardware control system, in particular the microprocessor unit isequipped with a UART-like serial special function shift register. 3-lineserial communication capabilities allow information to be loaded to thisspecial function register using the MOSI, MISO and CLK special pins onPORTB.

The particular implementation, a Serial Peripheral Interface (SPI) basedcommunication link, is part of a standard set of protocols used. Itsuffices to indicate that the link asserts information on the ATTiny26serial interface register. Once the desired information is clocked ontothe SERIAL register (USISR), a latch signal is triggered. The latchsignal is tied to the external interrupt INT0 pin (PORTB,6) which isconfigured in software accordingly. When the program handles theinterrupt request, the incoming data can be read directly from the USISRvariable and loaded into the on-board EEPROM.

The advantage of using the AVR's SPI capabilities is that the chip canbe both programmed and talked to via a parallel port, using 5V signalsan requiring no level conversion components as would be the case withRS-232 serial communication. The second advantage is that thecommunication is handled entirely within the External Interrupt Serviceroutine associated to external interrupt 0. This means that the systemcan be programmed while it is running, without having to reset theimages being switched onto the displayed. When the required instructionis just a READ STATE from the EEPROM or the current state array, theinterruption time causes no discernible glitches on the image beingdisplayed.

FIG. 20 illustrates an example prototype having 25 pixel module used inconjunction with a simple graphical user interface (GUI). It should benoted that the pixel is displaying the exact tone gradient on the GUI.

FIG. 21 illustrates a schematic circuit diagram of the power supply androuting box used in conjunction with the present invention. The fourmodule prototype is powered from a single power board. The board isconnected to the mains AC lines and contains two transformers, one forHV metallic isolation and one to step down the HV AC waveform to producea ˜20 V AC wave which is then rectified and fed through 12 V and 5 Vvoltage regulators. The isolated HV AC wave of approximately 156Vamplitude is rectified using a power diode bridge and the rectifiedoutput is connected to a 33 u electrolytic capacitor. A bleed resistoris placed in parallel to the capacitor, allowing it to discharge whenthe system is off.

The power board also contains the parallel port connector. Thecommunication signals are buffered and then directed to the 13-Pin MiniDin connector that will hook the board up with the actual displaypanels. The three power rails, HV, +12V and +5V are also routed to theconnector.

FIG. 22 illustrates a modular system using a prototype design consistingof four square modules of 25 pixels (each module labeled internally as‘1’, ‘2’, ‘3’, and ‘4’). Each pixel has dimensions of 9 cm a side. Theuse of four baseline modules exploits the idea of modularity andflexibility of design. These 25 pixel modules could themselves bearranged in a 2×2 matrix configuration, displaying forms or shapes, orif preferred in a 1×4 vector configuration to display information alongthe lines of alphanumeric characters. Therefore, while the finalaggregate design is a coordinated and integrated information displaysystem, it is actually made up of a network of these baseline modules.The motivation behind this feature is the lack of uniformity in thedesign of buildings and surfaces where this programmable window can beapplied.

The final prototype is intended to be the basic necessary hardware andfirmware platform to interface the switching circuitry 2204 to ahigher-level control entity such as a laptop computer 2202. Thisabstracted design requires only software-level changes to accommodatemany other applications such as the display of different successiveimages.

Another important design characteristic is the use of an adjacentswitchbox. Each baseline module is composed of a panel, on-panel signalbuses, and a peripheral switching circuit. Wire resistance and switchingcapacitances should be minimized, accounting for the on board locationof the routing or switching box—effectively as close to the pixels aspossible. The power and control signal lines are, in addition, routedusing 13-Pin Mini Din connectors and low resistance and power-losslines.

This need to have the control box adjacent to the switching panelimposes a constraint on its maximum size and explains the basis for theneed to resort to a solution based on integrated circuits. Given thesize constraint, the idea is to try and minimize hardware overhead. Theclearest choice is using a single microcontroller to run the switchingtimes, handle bi-directional communications, and implement the on boardimage update processes. Controlling the timing and switching in softwareeliminates the need for shift registers, digital comparators andadditional supporting ICs. A printed circuit board was also designed inorder to minimize the size of the on-board control circuits.

FIG. 23 illustrates an initial 4×4 test module being driven by theoptional MLA strategy. FIG. 24 illustrates a prototype having 25-pixelsub-modules.

Additionally, the present invention provides for an article ofmanufacture comprising computer readable program code contained withincontrolling one or more modules implementing shading control in a largescale transparent electronic display using SPD Film. Furthermore, thepresent invention includes a computer program code-based product, whichis a storage medium having program code stored therein which can be usedto instruct a computer to perform any of the methods associated with thepresent invention. The computer storage medium includes any of, but isnot limited to, the following: CD-ROM, DVD, magnetic tape, optical disc,hard drive, floppy disk, ferroelectric memory, flash memory,ferromagnetic memory, optical storage, charge coupled devices, magneticor optical cards, smart cards, EEPROM, EPROM, RAM, ROM, DRAM, SRAM,SDRAM, or any other appropriate static or dynamic memory or data storagedevices.

CONCLUSION

A system and method has been shown in the above embodiments for theeffective implementation of a programmable window—a device forcontrolling the opacity of small-scale areas within a large-scaletransparent membrane. While various preferred embodiments have beenshown and described, it will be understood that there is no intent tolimit the invention by such disclosure, but rather, it is intended tocover all modifications falling within the spirit and scope of theinvention, as defined in the appended claims. For example, the presentinvention should not be limited by hardware materials, software/program,computing environment, or specific computing hardware.

The above enhancements are implemented in various computingenvironments. For example, the present invention may be implemented on aconventional IBM PC, an equivalent, multi-nodal system (e.g., LAN), oron a networking system (e.g., Internet, WWW, wireless web). Allprogramming and data related thereto are stored in computer memory,static or dynamic, and may be retrieved by the user via: conventionalcomputer storage, display (i.e., CRT), and/or hardcopy (i.e., printed)formats.

1. A method to affect natural lighting profiles in a programmable windowmade from a single transparent membrane, said method comprising: forminga matrix comprising a plurality of rows and columns of pixel elementswithin said single transparent membrane, each pixel element comprising asuspended particle device having two conductive parallel plates havingorganic particles suspended between, each of said one or more pixelscapable of being driven to control opacity; setting, in a programmablecontroller, a first degree of shading in a first pixel in said matrix ofpixel elements and a second degree of shading in a second pixel in saidmatrix, said first degree of shading different in value than said seconddegree of shading; receiving one or more control commands from saidprogrammable controller to control opacity of said first and secondpixel elements in said matrix; in response to said one or more controlcommands, independently driving said first and second pixel within saidsingle transparent membrane to said first degree of shading and seconddegree of shading, respectively, said independent driving comprisingcontrolling alternating electric field between two conducting parallelplates of each pixel element and controlling a degree of transparency ofeach pixel according to respective control commands, said first andsecond degree of transparencies controlled according to strength andduration of the alternating electric field, said control done byaddressing a row and column corresponding to each of said first andsecond pixel and applying an appropriate voltage to create a specificvoltage difference at each of said first and second pixel, andcontrolling natural light entering said programmable window based oncontrolling transparency of individual pixels within said singletransparent membrane.
 2. A method to affect natural lighting profiles ina programmable window made from a single transparent membrane, saidmethod comprising: forming a matrix comprising a plurality of rows andcolumns of pixel elements within said single transparent membrane, eachpixel element comprising a suspended particle device having twoconductive parallel plates having organic particles suspended between,each of said one or more pixels capable of being driven to controlopacity; setting, in a programmable controller, a first degree ofshading in a first pixel in said matrix of pixel elements and a seconddegree of shading in a second pixel in said matrix, said first degree ofshading different in value than said second degree of shading; receivingat least one control command; said at least one control commandidentifying a first desired degree of transparency % T1 of said firstpixel element and a second degree of transparency % T2 of said secondpixel element; calculating a first desired voltage VRMS_(required1)between two conducting parallel plates of said first pixel element toachieve said first degree of transparency and calculating a secondvoltage VRMS_(required2) between two conducting parallel plates of saidsecond pixel element to achieve said second degree of transparency, saidcalculation based on at least one transformation function, saidtransformation function dependent on a desired degree of transparency,and addressing a column and row corresponding to each of said first andsecond pixel to independently drive them at the first and second desiredvoltage, wherein said first pixel is independently driven within saidsingle transparent membrane to said first degree of transparency bysetting the voltage between conducting plates of said first pixel toVRMS_(required1) and said second pixel is independently driven withinsaid single transparent membrane to said second degree of transparencyby setting the voltage between conducting plates of said second pixel toVRMS_(required2); controlling natural light entering said programmablewindow based on controlling transparency of individual pixels withinsaid single transparent membrane.
 3. A computer program product toaffect natural lighting in a programmable window made from a singletransparent membrane, said computer program product comprising acomputer readable storage medium having computer readable program codethat is executed in conjunction with a computer, said computer readablestorage medium comprising: computer readable program code receiving atleast one control command associated with a plurality of rows andcolumns of pixel elements within said single transparent membrane, saidat least one control command identifying a first desired degree oftransparency % T1 of a first pixel element within said singletransparent membrane and a second degree of transparency % T2 of asecond pixel element within said single transparent membrane; computerreadable program code calculating a first desired voltageVRMS_(required1) between two conducting parallel plates of said firstpixel element to achieve said first degree of transparency andcalculating a second voltage VRMS_(required2) between two conductingparallel plates of said second pixel element to achieve said seconddegree of transparency, said calculation based on at least onetransformation function, said transformation function dependent on adesired degree of transparency, and computer readable program codeaddressing a column and row corresponding to each of said first andsecond pixel to independently drive them at the first and second desiredvoltages, wherein said first pixel is independently driven within saidsingle transparent membrane to said first degree of transparency bysetting the voltage between conducting plates of said first pixel toVRMS_(required1) and said second pixel is independently driven withinsaid single transparent membrane to said second degree of transparencyby setting the voltage between conducting plates of said second pixel toVRMS_(required2); computer readable program code controlling naturallight entering said programmable window based on controllingtransparency of individual pixels within said single transparentmembrane.